1. Field of the Invention
The present invention relates to the field of laser technology, and more particularly to methods for generating high energy, ultra-short laser pulses.
2. Description of Related Art
Chirped pulse amplification (CPA) is very useful for producing ultra short-duration high-intensity pulses for use in high peak power ultra-short pulse laser systems. Material removal with such ultra-short pulses is especially useful for many purposes, as it is almost non-thermal and exerts essentially no pressure on the work surface. CPA increases the energy of an ultra-short laser pulse while avoiding optical amplifier damage. In this technique, the duration of the pulse is increased by first dispersing the ultra-short laser pulse temporally as a function of wavelength (a process called “chirping”) to produce a chirped pulse, then amplifying the chirped pulse, and then recompressing the chirped pulse to significantly shorten its duration. Lengthening the pulse in time (i.e., “stretching” the pulse) reduces the peak power of the pulse and, thus, allows energy to be added to the pulse without reaching a damage threshold of the pulse amplifier and optical components. The amount of pulse amplification that can be achieved is typically proportional to the amount of pulse stretching and subsequent compression. Typically, the greater the amount of stretching and compression, the greater the possible pulse amplification.
A fiber Bragg grating may be used for chirping a pulse and recompressing the pulse. However, the amount of stretching or compression by the fiber Bragg grating is substantially fixed by the physical dimensions of the fiber Bragg grating. Thus, various sizes of fiber Bragg gratings are used for chirping and/or compressing pulses. In some fiber Bragg gratings, nominal adjustments to the pulse length may be provided by physically, mechanically, or thermally stretching the optical fiber of the fiber Bragg grating to modify the length of the optical fiber. Unfortunately, the amount of adjustment to the pulse length is relatively small and the optical fiber may suffer damage from the physical stress and strain of the stretching. There is, therefore, a need for improved systems and methods of stretching and compressing optical pulses.
As noted, to bring the amplified pulse back to an ultra-short pulse length, the amplified pulse must be compressed. Typically, compression is done with bulk optical elements involving prism and grating pairs. Pulse-compression techniques of amplified chirped pulses are discussed, for example, in U.S. Pat. No. 5,822,097 by Tournois. Compact grating compressors present high third order dispersion, also referred to as dispersion slope. Such high order dispersion is mismatched to the dispersion parameters of standard fiber stretchers and prevents compression of a pulse below a few picoseconds.